Polyethylene is the most widely used commercial polymer. It can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the first method used to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE).
A common set-up of a production line for preparing low density polyethylene comprises a polymerization reactor, which can be an autoclave or a tubular reactor or a combination of such reactors, and additional equipment. For pressurizing the reaction components, which may comprise a set of two compressors, a primary compressor and a secondary compressor is used. At the end of the polymerization sequence, a production line for high-pressure polymerization may further includes apparatuses like extruders and granulators for pelletizing the resulting polymer. Furthermore, such a production line may also comprise means for feeding monomers and comonomers, free-radical initiators, modifiers or other substances at one or more positions to the polymerization reaction.
A characteristic of the radically initiated polymerization of ethylenically unsaturated monomers under high pressure is that the conversion of the monomers is often not complete. For every pass of the reactor or the reactor combination, only about 10% to 50% of the dosed monomers are converted in polymerizations in a tubular reactor and from 8% to 30% of the dosed monomers are converted in polymerizations in an autoclave reactor. The resulting reaction mixture usually leaves the reactor through a pressure control valve and is then may be separated into polymeric and gaseous components, with the unreacted monomers being recycled. To avoid unnecessary decompression and compression steps, the separation into polymeric and gaseous components may be carried out in at least two stages. The monomer-polymer mixture leaving the reactor can be transferred to a first separating vessel, frequently called high-pressure product separator, in which the separation in polymeric and gaseous components is carried out at a pressure that allows for recycling of the ethylene and comonomers separated from the monomer-polymer mixture to the reaction mixture at a position between the primary compressor and the secondary compressor. At the conditions of operating the first separation vessel, the polymeric components within the separating vessel are in liquid state. The liquid phase obtained in the first separating vessel is transferred to a second separation vessel, frequently called low-pressure product separator, in which a further separation in polymeric and gaseous components takes place at lower pressure. The ethylene and comonomers separated from the mixture in the second separation vessel are fed to the primary compressor where they are compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed and the joined streams are further pressurized to the pressure of the high-pressure gas recycle stream.
The polymerization process in a LDPE reactor is carried out at high pressures which can reach 350 MPa. Such high pressure may require special technology for the process to be handled in a safe and reliable manner. Technical issues in handling ethylene at high pressures are, for example, described in Chem. Ing. Tech. 67 (1995), pages 862 to 864. It is stated that ethylene decomposes rapidly in an explosive manner under certain temperature and pressure conditions to give soot, methane and hydrogen. This undesired reaction occurs repeatedly in the high-pressure polymerization of ethylene. The drastic increase in pressure and temperature associated therewith represents a considerable potential risk for the operational safety of the production plants.
A possible solution for preventing a drastic increase in pressure and temperature of this type involves installing rupture discs or emergency pressure-relief valves. WO 02/01308 A2, for example, discloses a specific hydraulically controlled pressure relief valve, which allows a particularly fast opening of the pressure relief valve in case of sudden changes in pressure or temperature. It is technically possible to handle such thermal runaways or explosive decompositions of ethylene within the polymerization reactor, however these situations are highly undesirable since thermal runaways or explosive decompositions of ethylene within the polymerization reactor lead to a shut-down of the polymerization plant with frequent emission of ethylene into the environment and loss of production.
Another threat to the operational safety of high-pressure polymerization plants is the occurrence of leaks. Due to the high pressure difference between the interior of the polymerization reactor and the surroundings, even small fissures in a wall of high-pressure equipment may lead to an exit of a considerably high amount of the reactor content resulting in locally high concentrations of combustible hydrocarbons in a short time period. On the other hand, in the case of larger leaks, the available time for reacting is extremely short.
Processes for polymerizing or copolymerizing ethylenically unsaturated monomers at pressures in the range of from 110 MPa to 500 MPa places specific demands on a reliable detection of combustible or explosive gases, which may leak from the polymerization equipment. Depending on the size and the position of the leak, the leakage rate may be extremely high or relatively low, with a risk of an accumulation of the leaked material. The leaked material may have a temperature in the range from 100° C. to 350° C. but can also be cold and may therefore sink to the ground and accumulate there. The concentration of leaked gas in a certain volume element in the vicinity of the polymerization plant can vary from substantially pure hydrocarbon to a very low concentration of combustible gas in air. Furthermore, the leakage can not only take place towards the atmosphere but can also occur at a section of the equipment which is covered by a cooling or heating jacket. Moreover, as such processes are not carried out in totally closed housings, also weather phenomena such as wind and rain may have an influence on the detection of leaked gases.
Another difficulty with respect to processes for preparing ethylene polymers at high pressure is that the reaction mixture is a supercritical composition comprising monomer and polymer. After a leakage of such a reaction mixture into the atmosphere, small polymer particles are formed which are subject to electrostatic charging. Consequently, there is an enhanced probability for an ignition after an explosive gas cloud has developed after an escape of reaction mixture.
WO 2008/148758 A1 discloses a method of operating a high-pressure ethylene polymerization unit comprising a tubular reactor equipped with a cooling jacket, in which the leakage of reaction mixture into the cooling jacket is controlled by monitoring the electrical conductivity of the aqueous cooling medium. Such a method, however, requires that at least one of the chemical substances in the reaction mixture changes the electrical conductivity of the aqueous cooling medium, and the method can only detect leakage at positions of the polymerization equipment which are covered by a cooling jacket.
It is standard practice to monitor gases by gas detectors. Gas detectors are devices that detect the presence of gases in an area, often as part of a safety system. This type of equipment is commonly used to detect a gas leak and can interface with a control system so a process can be automatically shut down. A gas detector can also sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave. Gas detectors can be used to detect combustible, flammable and toxic gases, and oxygen depletion. Common gas sensors include infrared point sensors, ultrasonic sensors, electrochemical gas sensors, and semiconductor sensors. More recently, infrared imaging sensors have come into use. These sensors are used for a wide range of applications and can be found in industrial plants, refineries, waste-water treatment facilities, vehicles, and homes.
One option for detecting the presence of hydrocarbons in air is the employment of catalytic detectors. Catalytic detectors operate by the principle that heat is generated in the oxidation of a combustible gas. The reaction takes place on the surface of a catalyst, with combustible gases reacting exothermically with oxygen in the air. The resulting temperature increase is converted into a sensor signal. Catalytic detectors have the disadvantages of being slow and requiring oxygen. When, for example by a larger gas escape, the air is immediately replaced by substantially pure hydrocarbon a catalytic detector is no longer able to recognize the hydrocarbon.
Accordingly there is a need to overcome these and other disadvantages and provide a process which allows for a very fast release of pressure after the occurrence of a leak in a high-pressure polymerization plant and avoids the build-up of explosive hydrocarbon gas/oxygen mixtures. The detection method should be very reliable and trustworthy and should be easy to implement in existing production lines for the preparation of low density polyethylenes.